1. Field of the Invention
The present invention relates to testing microelectronic packages, and more specifically, to methods and systems of simulating impedance variations and crosstalk effects during testing of printed circuit boards.
2. Description of Related Art
In many modern electronic systems, printed circuit boards and various other microelectronic packages are used to connect electronic components together for communication. A printed circuit board is typically a flat panel that interconnects electronic components using a pattern of flat conductive pathways, often referred to as traces, which are formed on a non-conductive substrate. A printed circuit board may contain conductive pathway patterns on the top and bottom surfaces of the printed circuit board or in layers through the interior of the printed circuit board. Conductive pathways on different layers of a printed circuit board interconnect through vias. Vias are conductive pathways that plate the walls of holes extending through the layers of the printed circuit board.
A single printed circuit board typically includes one or more conductive pathways between a transmitter and a receiver. A conductive pathway may include one or more traces, vias, or combinations thereof connected together for allowing electronic components, such as the transmitter and receiver, to propagate signals to one another using electronic conduction. The transmitter and receiver may be mounted to the printed circuit board and connected at designated portions of a trace pattern, often referred to as pads or lands. The transmitter and receiver may be connected to the printed circuit board using, for example, surface mount technology, through-hole mounting technology, or any other suitable technology as known to those skilled in the art. Surface mount technology connects electronic components to a printed circuit board by soldering electronic component leads or terminals to the top surface of the printed circuit board. Through-hole mount technology connects electronic components to a printed circuit board by inserting component leads through holes in the printed circuit board and then soldering the leads in place on the opposite side of the printed circuit board.
As printed circuit board data transmission speeds increase and transmission signals include frequency components with wavelengths comparable to the length of conductive pathways, it becomes necessary to use transmission line design techniques. The transition from lumped element behavior to transmission line behavior depends upon signal edge rates and on the total delay in the pulse transmission through a conductive pathway. In a lumped element mode, inductance and capacitance appear to the pulse to be concentrated at a point within the printed circuit board such that these factors do not need to be considered in design. On the other hand, in a transmission line mode, inductance and capacitance appear to be uniformly distributed throughout the interconnection, and as far as the pulse is concerned, the conductive pathway is infinite in length, and all the characteristics of wave propagation must be taken into consideration. Similar transmission line design techniques may be used for electrical characterization of conductive pathways in all microelectronic packages. Basic transmission line design parameters of interest include propagation delay, characteristic impedance, reflection coefficient, crosstalk, and risetime degradation.
Field failure of printed circuit boards can arise from variations between bulk-manufactured printed circuit boards and laboratory-tested printed circuit boards. Variations causing field failure can be due, for example, to the over-etch or under-etch of traces. As a result, the impedance of a particular conductive pathway in a bulk-manufactured printed circuit board can vary significantly among several bulk-manufactured printed circuit boards. For example, if the impedance variation in a trace of a bulk-manufactured printed circuit board is about 12%, a differential pair impedance having a nominal 100 ohm impedance at nominal value could be expected to vary between about 88 ohms and 112 ohms for testing purposes. When a printed circuit board is produced in a laboratory, it is most often aligned towards nominal impedance and has little manufacturing variation. Accordingly, there is a need for simple and accurate methods for simulating manufacturing variations during laboratory testing. Particularly, there is a need for convenient methods for simulating the impact of printed circuit board impedance variations within a laboratory environment.
Crosstalk is another transmission line parameter of interest to be varied within a laboratory environment. Crosstalk is any phenomenon by which a signal transmitted on one conductive pathway or other source creates an undesired effect in another conductive pathway. Accordingly, there is a need for convenient methods for simulating the effects of crosstalk on a conductive pathway of a printed circuit board during testing.